In the 1.0 and 1.1 models, Cloud 9 have put a thermal fuse in series with each heater element which is a very good idea. Therefore if one heater element is not working, then it could be that one of the thermal fuses has failed.
In the 1.2 models there is a thermal fuse on one of the plates and not on the other. This thermal fuse is wired to the PCB separately, rather than in series with the heater.
All the models are configured for either 70ohm heaters or 160ohm heaters. It is important heaters are replaced with the correct resistance part. The heaters are not marked, so you need to be careful that you replace heaters with the correct resistance. From the outside there is no way to tell which resistance heaters are required - you need to measure a working heater's resistance.
So far we've not had a problem using GHD 4.2 Type 1 backing plates on normal wide Cloud 9 Irons.
The heater assemblies are dismantled by pushing the heater out with a screw driver. The thermal fuse and the backing plate comes out at the same time.
This is the original thermal fuse attached to the heater element. Note the plastic mount has different size lugs on the end - the larger one goes into the arm and the smaller one goes at the PCB cover end.
A repaired thermal fuse:
The plastic mount can be "snapped" back into place.